I found "The Lovers' Tasks" (or "The Elfin Knight") as in
80 English Folk Songs
collected by Cecil Sharp & Maud Karpeles during the years 1916 to 1918. I really love this song and much prefer it to the similar "Scarborough Fair" as I find it more tuneful and the lyrics more intriguing.
I used to love singing this song with Kerstin Knie whereby we would share the verses on a gender basis. The song is of lovers giving each other impossible tasks to perform, the tasks being preconditions for love and marriage.